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May 15, 2006

Largest-ever 3D Map of a Million Galaxies

by Sam Savage

An international team of astronomers has released new results on the Cosmos, based on the largest map of the heavens ever produced.

This massive atlas emphatically confirmed recent findings that the Universe is full of 'dark energy', a mysterious substance that makes up three-quarters of our Universe, together with 'dark matter' which accounts for most of the remaining quarter. Understanding this composition is now one of the most important problems facing the whole of science.

"We now have a precise view of what makes up our Universe, but little idea as to why," said Prof. Ofer Lahav, a member of the international team and the Head of the Astrophysics Group at University College London. "It is intriguing that the ordinary matter our bodies are made of and that we experience in everyday life only accounts for a few percent of the total cosmic budget."

Our Universe contains billions of galaxies of all shapes and sizes. In recent years astronomers have used increasingly large surveys to map out the positions of these galaxies, stepping their way out into the Cosmos.

The new cosmic map unveiled today is the largest to date -- a three-dimensional atlas of over a million galaxies spread over a distance of more than 5 billion light years. The findings confirm that we live in a Universe filled with mysterious dark matter and dark energy.

"We have analyzed the patterns in this map and discovered waves of structure over a billion light years across," said Dr. Chris Blake of the University of British Columbia, principal author of the study. "These waves were generated billions of years ago and have been vastly stretched in size by the expanding Universe."

Construction of the cosmic atlas was led by co-author Dr. Adrian Collister of the University of Cambridge, as part of his PhD work, using a novel Artificial Intelligence technique he developed with his supervisor Prof. Ofer Lahav.

"By using very accurate distances of just 10,000 galaxies to train the computer algorithm we have been able to estimate reasonably good distances for over a million galaxies," said Collister. "This novel technique is the way of the future."

The original 2-dimensional positions and colours of the one million galaxies were from the Sloan Digital Sky Survey. The precise observations of the 10,000 galaxy distances were made as part of an international collaboration between U.S., U.K. and Australian teams using data from the Sloan Digital Sky Survey and the Anglo-Australian Telescope.

By measuring the positions of galaxies, astronomers can unravel the balance of forces that govern our Universe: the force of gravity which pulls everything together, and the competing effect of the expanding Universe which smoothes things out. These cosmic forces have arranged the galaxy distribution into a complex network of clusters, filaments and voids.

"The galaxy map can tell us the amount of ordinary 'baryonic' matter relative to the amount of mysterious 'dark matter'," said co-author Dr. Sarah Bridle of University College London. "We have confirmed that over 80% of the material in the Universe consists of an invisible dark matter whose nature is not yet understood."

The cosmic atlas of a million galaxies will shortly be made freely available on the World Wide Web for the benefit of other researchers. This free exchange of data is an important feature of modern astronomy, since many discoveries are only possible when different observations are combined.

The key problem in mapping the cosmos is determining the distance to each galaxy. Researchers can measure these distances because as the Universe expands, the colour of each galaxy changes as their emitted light waves are stretched or "Ëredshifted'.

Traditionally, astronomers have needed to take a "spectrum" of each galaxy to determine this distance, splitting its light into many components to reveal sharp features with which to measure the amount of redshifting. This requires a time-consuming, individual observation of each galaxy.

The new cosmic map has been constructed using a novel technique focusing on a special class of galaxy whose intrinsic colour is very well known. For these "ËLuminous Red Galaxies' researchers can measure the amount of colour distortion, and hence the approximate distance of the galaxy, just by looking at digital images of the sky, without the need to obtain a full spectrum.

All that is needed to exploit the technique is accurate observations of a small sample of the galaxies. In this case, precise measurements of just 10,000 galaxies were used to produce the atlas of over a million galaxies. These techniques will be very important for future large astronomical projects such as the international Dark Energy Survey, scheduled to start in 2009, in which University College London and the universities of Portsmouth, Cambridge and Edinburgh are key partners.